Polyolefins are widely used because of their properties. Nevertheless, the applications for polyolefins could be extended if certain properties such as stiffness, strength and heat resistance were improved. While fillers can improve these properties, their use is limited because there does not exist a good method for dispersing the fillers and achieving the desired properties without concomitant loss of toughness. This is presumably due to the high levels of fillers needed and concomitant problems with dispersing the fillers in the polyolefin matrix. There is a need for an improved method to disperse clay filler into a polyolefin matrix.
U.S. Pat. Nos. 5,830,820; 5,906,955; 5,925,587; 6,034,187 and 6,110,858 provide supported catalysts for the polymerization of olefins. Low levels of these supported catalysts are then used to catalyze the polymerization of olefins and provide polyolefins with only low levels of the support material.
U.S. Pat. No. 6,252,020 provides for clay-filled compositions by bulk and suspension polymerization of vinyl monomers such as styrene in the presence of clay and catalysts such as peroxides. Neither the polymerization of olefins such as ethylene or propylene nor the use of transition metals as catalysts is described or suggested.
U.S. Pat. No. 4,473,672 describes a process for making polyolefin compositions with a variety of fillers such as graphite, carbon black, an aluminosilicate clay, mica, talc, vermiculite or glass fibers by pretreating the filler with an organic magnesium compound and then adding the resultant composition to a transition metal and subsequently initiating the polymerization with an organoaluminum compound.
U.S. Pat. No. 4,564,647 teaches a process for producing a filled polyethylene composition with a variety of fillers. The process is general with regard to fillers. Specifically mentioned are metals, metal oxides, metal carbonates, titanium dioxide, mica, glass beads, glass fibers, silica, alumina, silica aluminate and organic pigments among many others. The filler may take various forms, such as powder, granule, flake, foil, fiber and whisker. The catalyst component is a transition metal treated with either a magnesium or manganese compound or is a Group 4 cyclopentadienyl compound. Despite a very broad disclosure, there is no mention of clay and no indication of a method of exfoliating clay.
PCT Int. Appl. WO 01/30864 discloses a method for producing a nanocomposite polymer by use of an acid-treated, cation-exchanging layered silicate material. The reference teaches that the silicate material is acidified by contacting it with a Bronsted acid such as a mineral acid or an amine hydrochloride. This requires an extra step, which increases the cost and complexity of the process. We found that the acid can also a have deleterious effect on the yield of the polymerization process, particularly when a Ziegler-Natta catalyst is used instead of a metallocene complex.
It has been observed that the synthesis of polyolefin-silicate nanocomposites remains a synthetic challenge (Bergman et al., Chem. Commun. (1999) 2179). These workers attributed the difficulty to the sensitivity of the vast majority of olefin polymerization catalysts to Lewis bases and water. Therefore, they used late transition metal catalysts to attempt to polymerize ethylene in the presence of a synthetic fluorohectorite. The product formed was described as a rubbery polymer that was highly branched. Such a polymer is unsuitable for many applications because of difficulties in processing.
There is a need for a simple process for providing clay-filled compositions and, in particular, for polyolefin compositions containing exfoliated clay.